1. Field of the Invention
The invention relates to the field of optical transmitters, and in particular, to optical power control circuitry for parallel optical transmitters having vertical cavity surface emitting laser (VCSEL) arrays.
2. Background Information
High speed direct coupled (DC) parallel optical data transmitters commonly use semiconductor vertical cavity surface emitting laser diode arrays (VCSEL""s) as their light sources. The laser device called a VCSEL (Vertical Cavity Surface Emitting Laser) is a semiconductor laser made of many layers, e.g., 600, which emits light vertically from a lower surface and in a direction parallel to the direction of its optical cavity, as opposed to an edge-emitting type laser structure. VCSEL""s have advantages over edge-emitting type structures because, for example, the edge-emitting type lasers must be precisely broken or cleaved individually to form each device during manufacturing. However, with VCSEL""s, literally millions of laser devices can be made simultaneously in an etching process.
VCSEL""s are currently some of the smallest lasers being produced. There is a relatively new type of VCSEL in development, the QD-VCSEL. The xe2x80x98QDxe2x80x99 signifies the Quantum Dots which are used in the active layer of this type of VCSEL. The QD-VCSEL promises to achieve even further size reductions.
VCSEL""s have a range of uses. For example, a specially designed VCSEL has been used to create an optical latch or optical state memory, the VCSEL transitioning and latching in the ON state when an optical input is received. Arrays of such VCSEL""s open up possibilities for various massively parallel optical computing applications such as pattern recognition. VCSEL""s have data communications applications as well as would be clear to one skilled in the art. For more information about VCSEL""s, see, for example, xe2x80x9cLASERS, Harnessing the Atom""s Light,xe2x80x9d Harbison et al., Scientific American Library, 1998, pages 169-177.
A graph representing the optical power output PO in milliwatts (mW) vs. the current input I in milliamps (mA) for a typical VCSEL is shown in FIG. 1. As is shown in the graph, the VCSEL does not begin lasing until the current through it exceeds a certain laser threshold value, shown as Ith in the figure. The slope of the curve above Ith is commonly referred to in the art as the differential quantum efficiency (DQE) of the VCSEL.
However, these two VCSEL diode parameters, Ith and DQE, along with the wavelength of the output light, are dependent on operating temperature, as well as on process variations. The manufacturing process variations are, at present, not completely controllable or predictable. Therefore, a method to adjust the current through the VCSEL to compensate for these variations is required. Some methods are known, for example, from U.S. Pat. No. 4,709,370, Bednarz et al., Nov. 24, 1987 and U.S. Pat. No. 3,633,120, Battjes Jan. 4, 1972.
To prevent over-powering the laser and to meet end of life requirements, a method must exist to compensate for the effects of process and operating temperature variations on output power.
Serial optical data transmitters ordinarily use laser diodes, e.g., VCSEL""s or edge-emitters, that are packaged with a photo detector that feeds back a current proportional to the optical output power of the laser diode. FIG. 2 shows a simple circuit integrating this type of laser/photo detector package into an operational amplifier (OP AMP) negative feedback loop to control the optical output power of the laser diode. During calibration, the feedback current through the photo detector is used to adjust the average optical power out of the laser by adjusting the potentiometer labeled xe2x80x98R POTxe2x80x99 changing the voltage on the non-inverting input of the OP AMP which controls the laser drive transistor to provide more or less current through the laser. In operation, this feedback current serves to dynamically adjusts the laser current in response to average optical power changes caused by changes in operating temperature.
However, to use this simple method for VCSEL diode arrays would require a photo detector for each VCSEL diode in the array. VCSEL arrays used in communications, for example, commonly contain 12 or more VCSEL""s, therefore 12 or more photo detectors would be required. Some problems with such an arrangement are that optical cross-talk from one VCSEL to adjacent VCSEL""s, and that physical size limitations may preclude using a photo detector for each VCSEL in an array. In recently contemplated applications of VCSEL""s, such as massively parallel processing, mentioned above, where perhaps millions of VCSEL""s would be used at once in an array, these problems could become overwhelming.
A known solution to these problems is to dynamically adjust all the VCSEL""s optical power levels using a reference VCSEL and reference photo detector in a VCSEL array, as shown in FIG. 3. Examples of such an arrangement are described in U.S. Pat. No. 5,625,480, Swirhun et al., issued Apr. 29, 1997 and U.S. Pat. No. 5,521,736, Swirhun et al., issued May 28, 1996. These prior patents describe (Abstract) electronic circuits and methods to dynamically compensate for the effects of the substrate temperature and aging behavior of the light emitters at both the transmitter and the receiver in a parallel optical interconnect system transmitting a plurality of DC non-return to zero (NRZ) data and an independent clock signal. An arrangement of light emitters is used to reduce or avoid skew problems as well.
The reference VCSEL and reference photo detector methods, however, are based on the assumption that all VCSEL""s in the array have identical Ith and DQE. Of course, if these parameters are not identical, which is likely, some VCSEL""s could be operating close to Ith. This could cause turn on delay and increased skew between channels, which, of course, is not a desirable phenomenon, especially as data rates increase. On the other hand, some VCSEL""s could be operating at higher average power than the reference VCSEL, possibly causing an over-power condition. Therefore, the reference VCSEL/photodetector methods are not a perfect solution to the problems discussed at the outset.
Another semiconductor laser diode control method is described in U.S. Pat. No. 5,019,769, Levinson, issued May 28, 1991. This patent describes (Abstract) a laser diode controller using a programmed micro-controller to accurately control the process of turning on and selecting the operating point of a laser diode. The laser diode has a front facet for transmitting light, and a back facet for monitoring the laser diode""s optical output power. Once the back facet of the laser diode is calibrated, the controller can accurately monitor the laser diode""s operating characteristics, and can select the best operating point current based on the current operating characteristics of the laser diode. During calibration of the laser diode, the controller can check the linearity of the laser diode""s optical output power as a function of drive current, and can thereby detect defects in the laser diode. In a full duplex optical link, the controllers at the link""s ends prevent the laser diodes from generating light at their full normal intensity until the integrity of the link has been established, thereby preventing light from the laser diode""s from accidentally damaging user""s eyes. Furthermore, the controllers can use the full duplex link to establish lower operating point drive currents that would otherwise be used to lengthen the lifetime of the laser diodes. A laser diode""s operating characteristics change over time in such a way as to enable the controller to predict when the laser will fail. The controller records the operating characteristics of the laser diode in a nonvolatile memory, analyzes changes in those characteristics, and generates a failure warning message when those changes match predefined failure prediction criteria.
However, this micro-controller arrangement is apparently just a more complex version of the prior method shown in FIG. 2 and described above, with the same disadvantages associated with having a photo detector for each laser in an array for a parallel communication system.
Therefore, a need exists for a better way of controlling laser optical power in a parallel array of optical transmitters, in particular, VCSEL""s.
Copending application Ser. No. 09/218,340, filed Dec. 22, 1998, assigned to the assignee of the present application, describes a constant current source circuit with variable temperature compensation to provide constant current to stabilize the performance of a load, e.g., a parallel array of VCSEL""s. The constant current source compensates for changes in the load due to temperature so that constant direct current bias power will be delivered to the load. Compensation for changes in performance resulting from changes of temperature is provided. The circuit mixes variable amounts of current having a negative temperature coefficient with current having a positive temperature coefficient. Analog and digital versions of the circuit are disclosed. In the analog version, the amount of current having a positive temperature coefficient is added to an amount of current having a negative temperature coefficient as determined by the voltage difference between a variable control voltage input to transistors and a bandgap reference voltage. A transistor in each of two current selectors is connected to the variable control voltage, one of which is connected to ground and the other of which is output; and another transistor in each current selector is connected to the reference voltage, and again one transistor is grounded and the other is output whose current is mixed with the output from the transistor in the first current selector connected to the variable control voltage. A continuous range of temperature coefficients are realizable by varying the control voltage with respect to the bandgap reference voltage. The digital version has a digital-to-analog converter connected to a bias voltage from the current having a positive temperature coefficient and a second digital-to-analog converter connected to a second bias voltage from the current having a negative temperature coefficient. A digital input signal to a corresponding switch determines if its respective transistor in each of the digital-to-analog converters conduct current. The two digital-to-analog converters may be configured in a common centroid arrangement of integrated complementary unit cells. The constant current source circuit can be used to drive off-chip parallel loads such as VCSEL""s.
Copending related application Ser. No. 09/388,313, filed Sep. 1, 1999, entitled DUAL CURRENT SOURCE WITH TEMPERATURE COEFFICIENTS OF EQUAL AND OPPOSITE MAGNITUDE,xe2x80x9d assigned to the same assignee as the present application, discloses (Abstract) a dual current source circuit which provides dual currents of the same magnitude and having coefficients of temperature compensation that are also equal but opposite.
Copending related application Ser. No. 09/429,280 filed Oct. 28, 1999, entitled xe2x80x9cVERTICAL CAVITY SURFACE EMITTING LASER (VCSEL) DRIVER WITH LOW DUTY CYCLE DISTORTION AND DIGITAL MODULATION ADJUSTMENTxe2x80x9d assigned to the same assignee as the present application, describes a method and apparatus which achieves low duty cycle distortion and digital modulation adjustment in one embodiment disclosed therein, by providing circuitry which provides for independent adjustment of currents to adjust for current source mismatches.
However, a need exists for an optical power adjustment arrangement which overcomes the limitations of the prior solutions. A need exists for optical power adjustment circuitry for parallel optical transmitters which can independently digitally adjust the threshold current and modulation current for each VCSEL in an array, as well as provide a global temperature coefficient for all the VCSEL""s in the array which can be digitally set. In particular, an open loop method would be desirable to avoid the disadvantages associated with closed loop methods described above.
It is, therefore, a principle object of this invention to provide a method and apparatus for optical power adjustment circuits for parallel optical transmitters.
It is another object of the invention to provide an open loop method to adjust the optical power level to compensate for temperature and process variations, particularly, in VCSEL arrays.
It is another object of the invention to provide a method and apparatus that solves the above mentioned problems by providing optical power adjustment circuitry for parallel optical transmitters which can independently digitally adjust the threshold current and modulation current for each VCSEL in an array, as well as providing a global temperature coefficient for all the VCSEL""s in, the array which can be digitally set.
These and other objects of the present invention are accomplished by the method and apparatus disclosed herein.
As mentioned above, others have solved the above-mentioned problems by including a reference VCSEL and photo detector on the VCSEL array. The characteristics of the reference VCSEL are used to set the low and high optical power levels of the remaining VCSEL""s in the array. According to an aspect of the present invention, however, neither an extra VCSEL as a reference, nor a photo detector, are required.
Further, according to another aspect of the invention, differences in Ith and DQE across a VCSEL array are compensated for, for each VCSEL, as compared with the prior solution of reliance on a single reference VCSEL and detector.
According to an aspect of the invention, a circuit to independently digitally adjust the threshold current and modulation current for each VCSEL in an array is provided.
According to another aspect of the invention, a global temperature coefficient (tempco) can be digitally set that applies to all the VCSEL""s in the array.
According to an aspect of the invention, a parallel optical transmitter that incorporates the invention contains a shift register that holds the digital code for a threshold adjustment digital to analog converter (DAC) and a modulation adjustment DAC.
According to an aspect of the invention, another shift register holds the digital code for a temperature coefficient (tempco) DAC current reference.
According to an aspect of the invention, a non-volatile storage device, e.g., an electronically eraseable and programmable read only memory (EEPROM), is provided which retains the desired digital codes when power is removed from the parallel transmitter.
According to another aspect of the invention, two currents with specified temperature coefficients are generated by the tempco DAC reference and input into the threshold adjustment DAC and the modulation current adjustment DAC.
According to an aspect of the invention, the threshold adjustment current DAC and the modulation current adjustment DAC both multiply their input currents by factors set by their respective digital input codes.
According to an aspect of the invention, the current out of the threshold current adjustment DAC is input to a laser current switch, along with the current out of the modulation current adjustment DAC.
According to an aspect of the invention, differential input data signals to the laser current switch (DATA +/xe2x88x92) cause the VCSEL optical power to switch between a low level, set by the threshold current adjustment DAC, and a high level, set by the addition of the threshold current adjustment DAC and the modulation current adjustment DAC.
According to an aspect of the invention, the tempco current DAC varies the low current out of the laser current switch with a tempco that is equal in magnitude to the VCSEL Ith vs. T characteristic.
According to an aspect of the invention, the tempco current DAC also varies the modulation current out of the laser current switch with a tempco that is equal in magnitude but opposite in sign to the DQE vs. T characteristic.
According to an aspect of the invention, as a result, the VCSEL outputs maintain constant low and high optical power levels across the desired temperature range.
According to an aspect of the invention, to determine the proper DAC settings, a serial digital bit stream is shifted into a shift register. The low and high optical power levels are then measured with an optical power meter. This procedure is repeated until the desired low and high optical power levels are reached for each VCSEL in the array.
According to an aspect of the invention, the temperature coefficient DAC""s can be set in two ways. The first way is to measure the tempcos of a sample of VCSEL arrays, and use the mean tempcos (Ith and DQE) for all VCSEL arrays thereafter. The second way is to use a thermocouple to determine the average tempcos for a given VCSEL array, and set the tempco DAC""s to have these tempcos. Therefore, each VCSEL array would have its own tempcos set independent of any other VCSEL array.
According to an aspect of the invention, once the proper digital codes are determined for the threshold current tempco, the modulation current tempco, the threshold current DAC, and the modulation current DAC, the digital code in the shift register can be written into the non-volatile memory, e.g., EEPROM.
These and other aspects of the invention will become apparent from the detailed description set forth below.